1. Field of the Invention
The present invention relates to an antenna for transmitting and receiving radio signals which is suitable for use with a portable apparatus (e.g., portable telephone set) and a radio (AM and FM) and TV apparatus using the same antenna, and more specifically to a small-sized antenna for transmitting and receiving radio signals of two or more frequency bands and a radio apparatus using the same small-sized antenna.
2. Description of the Prior Art
Conventionally, as an antenna for a radio apparatus such as a portable telephone set, a rod-shaped antenna 32 as shown in FIG. 24 has been used. This antenna 31 can be inserted into a casing of the radio apparatus 31 when not used but can be extended from the casing when used. Further, this rod antenna 32 is formed in such a way that the length thereof matches about a 1/4 wavelength or a 1/2 wavelength of radio signals to be transmitted and received. Therefore, when the transmitted and received frequency band is decided, the length of the rod antenna can be decided unequivocally. As a result, when low frequency signals are transmitted and received, the length of the antenna is inevitably lengthened. In order to shorten the external dimension of the rod antenna, various methods have been so far adopted such that an antenna wire (e.g., piano wire) is wound into a coil shape and the outer circumference thereof is covered with a resin, for instance.
On the other hand, in the antenna used for the portable apparatus, various methods have been so far adopted to prevent the long antenna from being obstructive when carried. For instance, when not used, the antenna is inserted into the portable apparatus in such a way that a part of the antenna is kept exposed from the casing to receive only a call signal or that another antenna for receiving only a call signal is attached to the casing and the entire antenna is extended to the outside from the casing to increase the sensitivity only during communications. Here, in the case where an antenna for receiving only a call signal and another antenna extended only during communications are both used, there are two types. One is a top coil type in which the call signal receiving antenna is loaded at the end of the communication antenna, and the other is a bottom coil type in which the call signal receiving antenna is always kept housed in the radio apparatus casing as it is even when the communication antenna is extended for use.
In summary, the call signal receiving antenna itself must be long enough to satisfy a length of about 1/4 or 1/2 wavelength of the transmitted and received signals, and in addition must be short enough not to be obstructive when carried. Therefore, in general, the call signal receiving antenna is wound into a coil shape to shorten the external length of the antenna. Further, in the case of the communication antenna, when the frequency band of the transmitted and received signals is low, since the length thereof increases and thereby the handling is not convenient, the communication antenna is usually shortened by winding it into a coil shape.
As described above, in the case where the antenna wire is wound into a coil shape in order to shorten the external length of the antenna, when the coil intervals are large, although no problem arises with respect to the electrical relationship between the coiled antenna elements, the coil length cannot be shortened sufficiently. On the other hand, when the coil intervals are short (the coil is wound densely), although the coil length can be shortened, since the current components perpendicular to the longitudinal direction of the antenna increase, the radiation resistance of the antenna is reduced due to the relationship with respect to the polarized wave plane. In this case, there exists a problem in that the antenna performance deteriorates even if a matching circuit is attached to the antenna.
Further, when the length of the antenna is about 1/4 wavelength of the transmitted and received signals, since this antenna can function as an antenna of about 3/4 wavelength of another frequency band three times higher than the frequency band of this antenna, this antenna can function in the same way as with the case of an antenna of about 1/4 wavelength. Therefore, it is possible to transmit and receive the frequency bands odd-times (e.g/., three times, five times, etc.) higher than the frequency band of this antenna by use of the same antenna. On the other hand, however, in the case where the frequency band is twice higher than the frequency band of this antenna, since the antenna length is about 1/2 wavelength thereof, this antenna cannot function as an antenna as far as a special matching circuit for 1/2 wavelength thereof is not attached thereto on the antenna feeding side. That is, it is impossible to transmit and receive signals of frequency bands of even-number (e.g., two times, four times, etc.) relationship with respect to the frequency band of this antenna or signals in the vicinity of these frequency bands by use of the same antenna. In practice, however, in the case of the portable telephone sets in Europe, for instance, since the frequency band of 900 MHz is used in GSM (group special mobile) (which corresponds to PDC (personal digital cellular) in Japanese system), and further the frequency band of 1800 MHz is used in DCS (digital cellular system) (which corresponds to PHS (personal handy-phone system) in Japanese system), it is particularly preferable to transmit and receive radio signals of a plurality of frequency bands by use of only a single antenna. Conventionally, however, in order to transmit and receive radio signals of both the frequency bands, it has been so far necessary to provide two different antennas or to use an antenna such that another antenna is connected to an end of a high frequency band antenna via a trap circuit in such a way that the total antenna length can match that of the low frequency band antenna.